Merge branch 'fix-calendar-with-missing-dates'

Fix a bug in the calendar export code, that breaks for events without an
end time ("until_time").
1 job for master in 1 minute and 42 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #1613
make_test

00:01:42

 
Name Stage Failure
failed
make_test Test
Ran 195 tests in 16.097s

FAILED (errors=18)
Creating test database for alias 'default'...
System check identified no issues (0 silenced).
Destroying test database for alias 'default'...
make.d/test.mk:31: recipe for target 'test_py' failed
make: *** [test_py] Error 1
ERROR: Job failed: exit code 1